Database query

Hi group, I am still getting to grips with PHP and MySQL and have a question that hopefully someone can easily answer: I have a search page which consists of two drop down menus, the user has 3 ways of finding information. The first is by selecting a location and a discipline which would look in the database and return any entries with the data in the two seperate fields. The second is where they can select the location only, and this will return all entries with that location The third is where they can select the discipline only, and again this will return all entries with that discipline. The problem I have is when the user selects the location and a discipline which should return no results as it is not an exact match it is returning results as it contains part of the information. My sql query is as follows: $sql = "SELECT * FROM vacancy WHERE ('$related_discipline' = related_discipline and '$location' = location) or ('$related_discipline' = related_discipline) or ('$location' = location) ORDER BY ID LIMIT $limitvalue1, $limit"; Could someone tell me what I need to change? Again help is always appreciated.